Technical development of the organic electroluminescent (EL) element is expected as next-generation displays and lighting. The features have advantages of low energy consumption, being capable of making thinner, excellent response speed, being capable of clear image display in both dark and bright places, and the like.
The basic structure of the organic EL element is a sandwich-like structure in which an organic compound of sole layer or multiple layers is sandwiched by a pair of electrodes. Specifically, there is proposed an element having a structure which uses, as a main configuration, a sandwich structure of a cathode/electron transport layer/emission layer/positive hole transport layer/anode/glass substrate, and which is obtained by appropriately adding a positive hole (electron) injection layer, buffer layer, interlayer insulating film, and the like in order to further enhance the properties. The emission layer which is a center of the sandwich structure uses various emitting materials, and the properties of the emission layer are required to easily flow electrons and positive holes which are transported from the cathode and anode, to have excellent light emission efficiency, to be durable, and the like.
Because of those demanded properties, development of phosphorescent materials has been required instead of the fluorescent materials having been conventionally applied as the emitting materials for the organic EL element. Since a generation probability ratio of excited molecule of an excited singlet to that of an excited triplet is 1:3 in the organic EL element, the phosphorescent material which exhibits phosphorescence by transition from the excited triplet state to the ground state is focused on in contrast to the fluorescent material which emits light by transition from the excited singlet to the ground state. Various organometallic complexes have been developed as such phosphorescent materials, and for example, there has been proposed an organometallic complex, as represented by the following Formula, in which a ligand (C—N ligand) having a heterocyclic ring and a C—N structure, and a ligand such as β-diketone are coordinated with a metal atom such as platinum or iridium. Specifically, PTL 1 discloses an organoiridium complex having a ligand with two benzene rings (diphenyl diketone) as the β-diketone ligand (SO2, etc. in PTL 1). In addition, PTL 2 discloses an organoplatinum complex or the like having a ligand with two butoxy-substituted benzene rings (tetra-butoxydiphenyl diketone) as the β-diketone ligand (PTL 2, Formula [1-1]). The light emission efficiency of the organometallic complexes described in the aforementioned PTLs is enhanced by application of the ligands having benzene ring as the β-diketone ligand.
